An optical cavity resonator is an arrangement of mirrors that forms a standing wave cavity for resonating light waves.
Cavity ring-down spectroscopy (CRDS) is a highly sensitive optical spectroscopic technique that enables very precise measurement of samples that scatter and absorb light. CRDS is used to study gas, liquid, and dispersed aerosol samples which absorb light at specific wavelengths. CRDS can be used to determine mole fractions of analytes down to the parts per trillion level.
A CRDS setup measures how long it takes for the light to decay to 1/e of its initial intensity, and this “ring-down time” can be used to calculate the concentration of the absorbing substance present in the cavity. The laser is then turned off in order to allow the measurement of the exponentially decaying light intensity leaking from the cavity. During this decay, light is reflected back and forth thousands of times between the mirrors, giving an effective path length for the extinction on the order of a few kilometers.
A typical CRDS setup consists of a laser and one or more highly reflective mirrors (generally two). The mirrors are placed in optical communication with each on opposite sides of a high-finesse optical cavity. If a sample placed in a cavity absorbs light, the amount of light decreases faster and the photon life is decreased.
The finesse of an optical cavity is represented as the total phase change (in radians) of the oscillating electromagnetic wave (i.e. the laser beam) over the time that the beam is trapped in the resonator. One exemplary equation that may be used to represent finesse is: 2π (effective path length)/(round-trip path length). Finesse equals π/(1−R) where R is the intensity reflectivity of the ring-down cavity mirror.
It is desirable to maximize the finesse value of an optical cavity in order to maximize the time a beam is trapped in the resonator, referred to as the photon lifetime. The longer the photon lifetime, the more sensitive the measurement.
In addition to finesse there are several other key performance parameters of a CRDS system known in the art. These measurements include wavelength coverage, dynamic range, detection limit, and sensitivity.
CRDS systems may be configured for specific experimental needs by structurally altering attributes including: (1) distance between mirrors in optical cavity; (2) the angle of reflection of each mirror; (3) the number of mirrors in a cavity; and (4) the preselected reflectivity value of each mirror. It is also known in the art to use dielectric films to increase the reflectivity value.
One problem known in the art is that mirrors for CRDS applications are custom manufactured and have preselected reflectivity value depending on the experimental application for which they are used. The reflectivity value cannot be readily altered after manufacturing resulting in a need for numerous custom mirrors to perform exacting experimentation techniques.
Another problem known in the art is that custom mirrors generally have an upper bound on their reflectivity. Additionally, commercially available mirrors generally have a maximum reflectivity value of 0.99999.
It is desirable to increase the maximum attainable reflectivity values available during an experimental process to allow CRDS measurements to be performed over a wider range of physical conditions.
It is further desirable to increase the key performance parameters attainable by a CRDS system known in the art, including finesse wavelength coverage, dynamic range, detection limit, and sensitivity.